Aluminum-material anodization method

ABSTRACT

Provided is an anodic oxidation treatment method for an aluminum material including aluminum or an aluminum alloy capable of forming a porous anodic oxide film of a porous type at a treatment voltage of 10 V or more while suppressing manifestation of a crystal grain pattern to the extent possible. Specifically, provided is an anodic oxidation treatment method for an aluminum material for subjecting an aluminum material to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a target voltage of 10 V or more, to form an anodic oxide film on a surface of the aluminum material, the method including, as pre-treatment of the anodic oxidation treatment, subjecting the aluminum material to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a voltage of 6 V or less until an electrical quantity reaches 0.05 C/cm 2  or more, to form a pre-film on the surface of the aluminum material.

TECHNICAL FIELD

The present invention relates to an anodic oxidation treatment method for an aluminum material, including subjecting an aluminum material including aluminum or an aluminum alloy to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution at a predetermined voltage, to form a porous anodic oxide film on a surface of the aluminum material, and more particularly, to an anodic oxidation treatment method for an aluminum material capable of suppressing manifestation of a crystal grain pattern through anodic oxidation treatment to the extent possible.

BACKGROUND ART

Aluminum itself is liable to be attacked by an acid, an alkali, or the like. Therefore, in order to impart corrosion resistance, abrasion resistance, and the like to an aluminum material, the aluminum material is widely and generally subjected to anodic oxidation treatment for forming an aluminum oxide (Al₂O₃) film (anodic oxide film) on the surface of the aluminum material by applying a current in an electrolyte solution through use of the aluminum material as an anode. In addition, for example, in the case of anodic oxidation treatment using, as the electrolyte solution, an acid aqueous solution of sulfuric acid, oxalic acid, phosphoric acid, or the like, an anodic oxide film called a porous-type film is formed through the anodic oxidation treatment. The porous-type film is formed of a dense film called a barrier layer which is formed on an inner side (aluminum side) and a porous film called a porous layer which is formed on an outer side of the dense film and has many pores. First, the barrier layer is generated in an early stage of the anodic oxidation treatment depending on a treatment voltage, and then many pores are generated in the barrier layer, and the many pores grow to form the porous layer.

Meanwhile, in general, a pattern derived from crystal grains in the aluminum material (crystal grain pattern) is not visible to the naked eye on the aluminum material before the anodic oxidation treatment, but when the above-mentioned anodic oxidation treatment is performed, the crystal grain pattern manifests mainly owing to a difference in orientation of the crystal grains.

With regard to the crystal grain pattern on the aluminum material after the anodic oxidation treatment, there has been proposed a technology for intentionally allowing the crystal grain pattern regarded as a highly decorative pattern to manifest upon light reflection by clearly showing the difference in crystal orientation (for example, see Patent Literature 1). However, for example, in an application as a member for residence, such as a doorknob or a fence, a member for a bicycle, such as a handlebar or a crank, a member for a vehicle, such as a boarding door frame or an inner panel, a decorative member, such as an accessory or a watch, a member for an optical product, such as a reflector or a camera, a roll for printing, or the like, the outer appearance and uniformity of the member are important in some cases. A member having a remarkable crystal grain pattern is judged as having a poor outer appearance in some cases.

The problem of the crystal grain pattern on the aluminum material after the anodic oxidation treatment becomes more conspicuous in the case where the aluminum material has a high aluminum purity (Al purity) owing to the crystal grain having a large size. In addition, the problem becomes more conspicuous also in the case where the surface of the aluminum material is subjected to mirror treatment by a mirror processing method, such as cutting processing, such as burnishing processing, buff polishing, electropolishing, or chemical polishing.

In view of the foregoing, as a method of allowing the crystal grain pattern on the surface of the aluminum material after the anodic oxidation treatment as described above to be invisible, a method involving adjusting a cooling speed in casting of the aluminum material before the anodic oxidation treatment, or subjecting the aluminum material to processing such as cold forging, to reduce the size of the crystal grain in the aluminum material to be smaller than a visible size (about 100 μm) to thereby allow the crystal grain pattern to be apparently inconspicuous is conceivable.

However, there are some products having a limitation on a processing method for aluminum, and hence the crystal grain is limited in reducing its size. In addition, particularly in the case where the aluminum material is a material having a high Al purity or a material requiring heat treatment for its production, it is technically difficult to reduce the size of the crystal grain to 100 μm or less. In addition, even when the size of the crystal grain can be reduced, an aggregate of the crystal grains in the aluminum material may look the same as one large crystal grain in an outer appearance, and there is a difficulty in obtaining a uniform outer appearance.

Meanwhile, in Patent Literature 2, in order to prevent a rush mat-like wrinkle called a streak and granular treatment unevenness called plane quality unevenness, which are liable to be generated through chemical etching owing to a difference in orientation of crystal grains, there has been proposed production of an aluminum support for a lithographic printing plate improved in surface shape, involving, prior to anodic oxidation treatment, performing (1) desmutting treatment, (2) preliminary electrochemical surface roughening treatment in a hydrochloric acid aqueous solution with an electrical quantity of from 1 C/dm² to 300 C/dm² using an alternate current at a predetermined frequency, (3) electrochemical surface roughening treatment in a hydrochloric acid aqueous solution, and (4) etching treatment in a predetermined amount and/or desmutting treatment in a hydrochloric acid aqueous solution. However, the preliminary electrochemical surface roughening treatment performed in this method is not treatment for forming the anodic oxide film of a porous type through the anodic oxidation treatment, but etching treatment for surface roughening by electrochemically dissolving aluminum in a monobasic acid.

CITATION LIST Patent Literature

[PTL 1] JP 2005-097735 A

[PTL 2] JP 2001-011699 A

SUMMARY OF INVENTION Technical Problem

In view of the foregoing, the inventors of the present invention have made detailed researches and investigations on the cause of the manifestation of the crystal grain pattern through the anodic oxidation treatment. As a result, the inventors have found that, in the aluminum material after the anodic oxidation treatment, the shapes of pores at an interface between an aluminum metal (Al) and the barrier layer (Al₂O₃) differ depending on the orientation of the crystal grains. That is, the investigations made by the inventors have revealed the following: while the barrier layer is first formed in an early stage of film formation in the anodic oxidation treatment and then pores begin to open in the formed film, a difference in orientation of crystal grains causes a difference in timing at which the pores are generated; as a result, the many pores generated at the interface between the aluminum metal (Al) and the barrier layer (Al₂O₃) have a slight difference in shape and irregularity; and the resultant slight difference in the many pores is also reflected in the porous layer subsequently formed through growth of the many pores. Then, even when the slight difference in the many pores of the anodic oxide film thus formed is extremely small, such slight difference is emphasized when light is applied onto a surface, manifests as the crystal grain pattern, and causes the aluminum material after the anodic oxidation treatment not to have a uniform outer appearance.

Then, based on the result of the investigations, the inventors of the present invention have made further investigations on a method of uniformizing the many pores to be generated at the interface between the aluminum metal (Al) and the barrier layer (Al₂O₃) to the extent possible irrespective of the orientation of the crystal grains. As a result, the inventors have found that, when anodic oxidation treatment at a low voltage is preliminarily performed until an electrical quantity reaches a predetermined one prior to anodic oxidation treatment at a target voltage of 10 V or more, to form a pre-film having many fine and uniform pores on the surface of the aluminum material, a porous layer having pores of uniform shapes can be formed through the subsequent anodic oxidation treatment at the target voltage, and the manifestation of the crystal grain pattern can be suppressed to the extent possible in the aluminum material after the anodic oxidation treatment. Thus, the inventors have completed the present invention.

Accordingly, an object of the present invention is to provide an anodic oxidation treatment method for an aluminum material including aluminum or an aluminum alloy capable of forming a porous anodic oxide film of a porous type at a treatment voltage of 10 V or more while suppressing manifestation of a crystal grain pattern to the extent possible.

Solution to Problem

That is, according to one embodiment of the present invention, there is provided an anodic oxidation treatment method for an aluminum material for subjecting an aluminum material including aluminum or an aluminum alloy to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a target voltage of 10 V or more, to form a porous anodic oxide film on a surface of the aluminum material, the method including, as pre-treatment of the anodic oxidation treatment, subjecting the aluminum material to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a voltage of 6 V or less until an electrical quantity reaches 0.05 C/cm² or more, to form a porous pre-film on the surface of the aluminum material.

In the present invention, the aluminum material including aluminum or an aluminum alloy serving as a target of the anodic oxidation treatment is not particularly limited, and an aluminum material in which a crystal grain pattern manifests owing to crystal grains in the aluminum material when the aluminum material is subjected to the anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a target voltage of 10 V or more to form the anodic oxide film of a porous type on the surface of the aluminum material serves as the target. In particular, an example of the target may be an aluminum material having a high Al purity in which the crystal grain in the material has a size of 100 μm or more owing to the high Al purity and thus the crystal grain pattern is liable to manifest. For example, there may be given a high-purity aluminum material having a purity of 99.99% or more or the like. In addition, in such aluminum material, the crystal grain pattern is liable to manifest also in the case where the surface of the aluminum material is subjected to mirror treatment by a mirror processing method, Such as buff polishing, electropolishing, cutting processing, or chemical polishing. Therefore, the present invention is also effective for the aluminum material having its surface subjected to such mirror treatment.

It should be noted that, in the present invention, the “target voltage” as the treatment condition of the anodic oxidation treatment refers to a voltage to be applied in the anodic oxidation treatment to be performed for a predetermined purpose through use of a predetermined treatment bath as described below. For example, a direct current voltage of from about 10 V to about 20 V is generally applied when a corrosion-resistant film, a film for dyeing, a film for decoration, or the like is formed on the surface of the aluminum material through use of a treatment bath of a 10 wt % to 20 wt % sulfuric acid aqueous solution as the polybasic acid aqueous solution. In addition, a direct current voltage of from about 10 V to about 600 V is generally applied when a corrosion-resistant film, an abrasion-resistant film, a film for decoration, or the like is formed on the surface of the aluminum material through use of a treatment bath of a 0.01 wt % to 4 wt % oxalic acid aqueous solution as the polybasic acid aqueous solution.

In addition, in the present invention, the size of the crystal grain in the aluminum material is determined by, for example, a method generally called a “cutting method” involving polishing (for example, buff polishing) the surface of the aluminum material to expose a cross section, and then applying a corrosive liquid (for example, a tucker liquid, sodium hydroxide, or the like) onto the cross section to dissolve the surface of the cross section of a sample and allow the crystal grain to be visually seen, and subsequently taking an image of the cross section with a microscope or an inverted microscope, drawing, for example, about three lines having a constant length (for example, 50 mm or 20 mm) on the taken image, counting the number of crystal grains on the lines, dividing the line length (L) by the number of crystal grains (N) to determine the value of L/N, and defining the resultant value of L/N as the size (length) of the crystal grain.

In the present invention, in the pre-treatment prior to the anodic oxidation treatment at the target voltage, the aluminum material is subjected to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a voltage of 6 V or less until an electrical quantity reaches 0.05 C/cm² or more, to form a pre-film on the surface of the aluminum material.

Herein, examples of the polybasic acid constituting the treatment bath may generally include a mineral acid, such as sulfuric acid, phosphoric acid, or chromic acid, and an organic acid, such as oxalic acid, tartaric acid, or malonic acid. Of those, sulfuric acid, phosphoric acid, and the like are preferred by virtue of a high treatment speed. The concentration of the polybasic acid in the treatment bath using the polybasic acid (polybasic acid aqueous solution) may be the same as in the general anodic oxidation treatment. For example, in the case of sulfuric acid, the concentration of sulfuric acid is 10 wt % or more and 20 wt % or less, preferably 14 wt % or more and 18 wt % or less.

In addition, in the pre-treatment of the present invention, the anodic oxidation treatment needs to be performed until the electrical quantity reaches 0.05 C/cm² or more while the voltage is maintained at 6 V or less. When the voltage is increased to more than 6 V, it becomes difficult to reduce a difference in timing at which pores begin to open, the difference resulting from a difference in orientation of the crystal grains. As a result, when the anodic oxidation treatment is subsequently performed at a target voltage of 10 V or more, the crystal grain pattern manifests in some cases. In addition, when the electrical quantity in the pre-treatment does not reach 0.05 C/cm², many fine and uniform pores are not formed in the formed pre-film in some cases, even when the voltage in the pre-treatment is maintained at 6 V or less. When the anodic oxidation treatment is subsequently performed at a target voltage of 10 V or more, the manifestation of the crystal grain pattern cannot be prevented in some cases. Herein, the voltage in the pre-treatment does not have a particular lower limit, but when the voltage is 1 V or less throughout the pre-treatment, significant time is required for the formation of the pre-film in some cases. In addition, also the electrical quantity does not have a particular upper limit, but almost the same effect is obtained even when the electrical quantity is significantly increased. For example, the case of an electrical quantity of more than 5 C/cm² is not preferred because such electrical quantity offers a pre-film having a thickness of several micromillimeters or more in some cases, resulting in a waste of treatment time in the case of removing the pre-film in a subsequent step.

Herein, in the present invention, as the voltage in the pre-treatment, a constant voltage of 6 V or less may be applied from the beginning to the end of the pre-treatment. Alternatively, the voltage may be gradually increased within a range of 6 V or less from the beginning to the end of the pre-treatment. Further alternatively, the voltage may be gradually reduced within a range of 6 V or less from the beginning to the end of the pre-treatment. In addition, the treatment time period of the pre-treatment is a time period until the electrical quantity in the pre-treatment reaches 0.05 C/cm². Further, the treatment temperature of the pre-treatment may fall within a range of, for example, 5° C. or more and 35° C. or less in the case of using sulfuric acid, as in the general anodic oxidation treatment.

In the present invention, the pre-film to be formed in the pre-treatment has a large number of fine pores whose pore shapes are entirely uniform as compared to the anodic oxide film to be formed in the anodic oxidation treatment at the target voltage. In addition, the thickness of the pre-film is, for example, roughly 25 nm or more in the case of using a 15 wt % sulfuric acid aqueous solution, while the thickness varies depending on the kind, concentration, and the like of the polybasic acid in the polybasic acid aqueous solution to be used as the treatment bath.

In the present invention, the pre-film to be formed in the pre-treatment has a large number of fine pores suppressed in irregularity at the interface between the aluminum metal (Al) and aluminum oxide (Al₂O₃) as compared to the anodic oxide film to be formed in the anodic oxidation treatment at the target voltage. Further, the shapes and sizes of the pores formed in the pre-film are constant and uniform irrespective of the orientation of the crystal grains because the pores are large in number and suppressed in irregularity. Thus, an anodic oxide film having relatively uniform pores can be formed in the subsequent anodic oxidation treatment at the target voltage (10 V or more), and the manifestation of the crystal grain pattern resulting from the difference in orientation of the crystal grains can be suppressed to the extent possible.

In the present invention, the anodic oxidation treatment at a voltage of 10 V or more after the pre-treatment may be performed in the same manner as in the related-art anodic oxidation treatment for forming an anodic oxide film of a porous type. Also the polybasic acid aqueous solution to be used as the treatment bath and treatment conditions may be the same as those in the related-art anodic oxidation treatment. A uniform anodic oxide film having relatively uniform pores and no crystal grain pattern can be formed in the anodic oxidation treatment at the target voltage (10 V or more).

In addition, in the present invention, the treatment bath to be used in the pre-treatment and the treatment bath to be used in the anodic oxidation treatment may be aqueous solutions of the same polybasic acid or of different polybasic acids. Further, the concentrations of the polybasic acids in the polybasic acid aqueous solutions may be the same as or different from each other. The use of polybasic acid aqueous solutions of the same kind and the same concentration in the pre-treatment and the anodic oxidation treatment has an advantage in that the need for exchange of the treatment bath is eliminated at the time of transition from the pre-treatment to the anodic oxidation treatment. In addition, for example, in the case where a polybasic acid aqueous solution exhibiting a relatively low treatment speed needs to be used as the treatment bath in the anodic oxidation treatment at the target voltage (10 V or more), the use of polybasic acid aqueous solutions of different kinds and/or different concentrations involving using a polybasic acid aqueous solution exhibiting a high treatment speed as the treatment bath in the pre-treatment can shorten the entire treatment time period.

Further, in the present invention, when the anodic oxidation treatment of the aluminum material at the target voltage (10 V or more) is performed, pre-film removal treatment for removing the pre-film formed in the pre-treatment may be performed as required during or after the anodic oxidation treatment.

Herein, an example of the pre-film removal treatment during the anodic oxidation treatment may be a method involving, when performing the anodic oxidation treatment of the aluminum material after the pre-treatment, performing the anodic oxidation treatment with an electrical quantity 50 times or more, preferably 80 times or more as large as the electrical quantity applied in the pre-treatment, to dissolve the porous pre-film formed in the pre-treatment in the treatment bath of the anodic oxidation treatment to thereby remove the porous pre-film. In such method, when the electrical quantity in the anodic oxidation treatment is an electrical quantity less than 50 times as large as the electrical quantity applied in the pre-treatment, the pre-film is insufficiently dissolved in the anodic oxidation treatment, and the porous pre-film is left undissolved and remains on the surface in some cases.

In addition, an example of the pre-film removal treatment after the anodic oxidation treatment may be a method involving immersing the aluminum material after the anodic oxidation treatment in an acid or alkaline aqueous solution, to chemically dissolve the porous pre-film remaining on a surface after the anodic oxidation treatment to thereby remove the porous pre-film.

When the porous pre-film formed in the pre-treatment is removed through the pre-film removal treatment during or after the anodic oxidation treatment as described above, a film having pores which have a uniform target pore diameter when seen from a surface and are each uniform from the bottom to the top of the pore can be obtained. That is, there is an advantage in that a film having a structure similar to that of a film obtained through treatment only at the target voltage (general anodic oxidation treatment) is obtained.

Further, when the anodic oxidation treatment of the aluminum material after the pre-treatment is performed, pre-film partial dissolution treatment for dissolving the porous pre-film formed on the aluminum material after the pre-treatment under treatment conditions in which 10% or more of a wall thickness of the porous pre-film remains may be performed. An example of the pre-film partial dissolution treatment may be a method involving treating the aluminum material after the pre-treatment under treatment conditions preliminarily determined by preparing a test sample which is substantially the same as the aluminum material after the pre-treatment, and determining treatment conditions in which 10% or more of a wall thickness between pores of the pre-film remains through use of the test sample. When the pre-film partial dissolution treatment is preliminarily performed to adjust the wall thickness between pores of the pre-film as described above, a porous anodic oxide film in which the pre-film does not remain on the surface and a porous layer is uniformly formed can be obtained.

When the porous pre-film is dissolved in the partial dissolution treatment until the wall thickness between pores of the pre-film reaches less than 10% of the wall thickness between pores of the pre-film at the time of its formation in the pre treatment, the porous pre-film becomes excessively fragile, and a base material is exposed at some positions at the time of the subsequent anodic oxidation treatment. Anodic oxidation preferentially occurs at the positions in which the base material is exposed, and hence a uniform anodic oxide film is not formed in some cases.

Advantageous Effects of Invention

According to the method of the present invention, the porous anodic oxide film of a porous type can be formed on the aluminum material including aluminum or an aluminum alloy at a treatment voltage of 10 V or more while the manifestation of a crystal grain pattern is suppressed to the extent possible. Therefore, an anodic-oxidation-treated aluminum material to be used for an application in which invisibility of the crystal grain pattern and uniformity of an outer appearance are important, such as a member for residence, a member for a bicycle, a member for a vehicle, a decorative member, a member for an optical product, or a roll for printing, can be easily produced on an industrial scale.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 includes a SEM photograph (upper photograph) of an upper portion of a cross section of a test piece obtained in Example 1 at a magnification of 3,000 times, and a SEM photograph (lower photograph) of an upper portion of a cross section of an anodic oxide film of the test piece at a magnification of 50,000 times.

FIG. 2 is a SEM photograph of an upper portion of a cross section of a reference test piece at a magnification of 100,000 times, the reference test piece being obtained by, in the anodic oxidation treatment of an aluminum material after pre-treatment obtained by forming a pre-film through the pre-treatment in Example 14, intermitting the anodic oxidation treatment 1 min after the beginning of the anodic oxidation treatment.

DESCRIPTION OF EMBODIMENTS

Now, a preferred embodiment of the present invention is more specifically described on the basis of Examples and Comparative Examples.

Examples 1 to 20

Plate materials having Al purities shown in Table 1 or plate materials of kinds shown in Table 1 were each used as an aluminum material. An aluminum piece having a size of 50 mm×50 mm×10 mm was cutout from each of the plate materials, and was subjected to mirror treatment by a mirror processing method shown in Table 1 until a surface roughness Rt of less than 200 nm was achieved. The resultant aluminum piece after mirror treatment was subjected to pre-treatment for forming a porous pre-film in a polybasic acid aqueous solution and under the treatment conditions shown in Table 1, and as well, subjected to anodic oxidation treatment at a target voltage in a polybasic acid aqueous solution and under the treatment conditions shown in Table 1, followed by washing with water and drying. Thus, aluminum pieces (test pieces) after anodic oxidation treatment of Examples 1 to 19 were obtained.

[Evaluation of Crystal Grain Pattern Through Surface Observation]

The test pieces obtained in Examples 1 to 20 were each evaluated for its crystal grain pattern through surface observation in which the case where a crystal grain pattern was seen in visual observation under fluorescent light having an illuminance of 1,500 Lux or more and 2,500 Lux or less was evaluated as “×”, the case where a crystal grain pattern was not seen in the visual observation under fluorescent light having an illuminance of 1,500 Lux or more and 2,500 Lux or less was evaluated as evaluated as “0”, and further, the case where a crystal grain pattern was not seen in visual observation under video light having an illuminance of 15,000 Lux or more and 20,000 Lux or less was evaluated as “⊚”.

The results are shown in Table 1.

[States of Pre-Film and Anodic Oxide Film Through SEM Observation]

In FIG. 1, the upper photograph is a SEM photograph of an upper portion of a cross section of the test piece obtained in Example 1 observed by SEM at a magnification of 3,000 times, and the lower photograph is a SEM photograph of an upper portion of a cross section of an anodic oxide film of the same test piece obtained in Example 1 at a magnification of 50,000 times. A pre-film disappeared by being dissolved in the anodic oxidation treatment, and a uniform anodic oxide film of a porous type was formed.

FIG. 2 is a SEM photograph of an upper portion of a cross section of a reference test piece observed by SEM at a magnification of 100,000 times, the reference test piece being obtained by, in the anodic oxidation treatment of an aluminum material after pre-treatment obtained by forming a pre-film through the pre-treatment in Example 14, intermitting the anodic oxidation treatment 1 min after the beginning of the anodic oxidation treatment. A residual pre-film was observed on the upper surface of an anodic oxide film of a porous type. It should be noted that, in the test piece obtained in Example 14 in which the anodic oxidation treatment was performed under the condition of a treatment time period of 45 min, the residual pre-film was not observed on the upper surface of the anodic oxide film.

Example 21

A porous pre-film was formed through pre-treatment in a treatment bath of 15 wt % sulfuric acid (18° C.) under the conditions of a voltage of 5 V and an electrical quantity of 0.1 C/cm² in the same manner as in Examples 1 to 20 described above, and then a porous anodic oxide film was formed through anodic oxidation treatment in the same treatment bath of 15 wt % sulfuric acid (18° C.) under the conditions (conditions of pre-film removal treatment) of a voltage of 15 V and an electrical quantity of 6 C/cm² (corresponding film thickness of 3 μm). Thus, an aluminum piece (test piece) after anodic oxidation treatment of Example 21 was obtained.

The test piece obtained was evaluated for its crystal grain pattern through surface observation in the same manner as in Examples 1 to 20. The result is shown in Table 1.

In addition, a cross section of the test piece obtained was observed by SEM, and as a result, it was confirmed that the pre-film did not remain on an upper portion of the film and the film had a uniform structure. No significant difference was observed in outer appearance as compared to Example 1 in which the pre-film remained.

Example 22

A porous pre-film was formed through pre-treatment in a treatment bath of 15 wt % sulfuric acid (18° C.) under the conditions of a voltage of 5 V and an electrical quantity of 0.1 C/cm² in the same manner as in Examples 1 to 20 described above, and then a porous anodic oxide film was formed through anodic oxidation treatment in the same treatment bath of 15 wt % sulfuric acid (18° C.) under the conditions of a voltage of 15 V and an electrical quantity of 2 C/cm². After the electrical quantity reached 2 C/cm², a sample was continuously left immersed in the same bath for 15 min (pre-film removal treatment) and then taken out therefrom. Thus, an aluminum piece (test piece) after anodic oxidation treatment of Example 22 was obtained.

The test piece obtained was evaluated for its crystal grain pattern through surface observation in the same manner as in Examples 1 to 20. The result is shown in Table 1.

In addition, a cross section of the film of the test piece obtained was observed by SEM, and as a result, it was confirmed that the pre-film did not remain on an upper portion of the film and the film had a uniform structure. The outer appearance was almost the same as in the case of not performing the pre-film removal treatment.

Example 23

Two aluminum pieces after pre-treatment each having formed therein a porous pre-film were each prepared by subjecting an aluminum piece after mirror treatment subjected to the same mirror treatment to the same pre-treatment in a treatment bath of 15 wt % sulfuric acid (18° C.) under the conditions of a voltage of 5 V and an electrical quantity of 0.1 C/cm² in the same manner as in Examples 1 to 20 described above.

One of the resultant aluminum pieces after pre-treatment was used as a test sample, and the test sample was immersed in a 10 wt % phosphoric acid aqueous solution (20° C.) for 2 min (pre-film partial dissolution treatment). The test piece was subjected to surface observation with an electron microscope, and as a result, it was confirmed that the wall thickness between pores of the pre-film was reduced to 15% as compared to the aluminum piece after pre-treatment without the partial dissolution treatment.

Next, the aluminum piece after pre-treatment without the partial dissolution treatment was subjected to pre-film partial dissolution treatment under exactly the same conditions as above, and then subjected to anodic oxidation treatment in a treatment bath of 15 wt % sulfuric acid (18° C.) under the conditions of a voltage of 15 V and an electrical quantity of 2 C/cm² without confirming the wall thickness between pores of the pre-film, to form a porous anodic oxide film. Thus, an aluminum piece (test piece) after anodic oxidation treatment of Example 23 was obtained.

The test piece obtained was evaluated for its crystal grain pattern through surface observation in the same manner as in Examples 1 to 20. The result is shown in Table 1.

In addition, a cross section of the film of the test piece after anodic oxidation treatment was confirmed with an electron microscope, and as a result, the pre-film, which was observed in an upper portion of the film of the test piece of Example 1, was not observed, and it was confirmed that the film had a uniform structure.

Comparative Examples 1 to 10

Plate materials having Al purities shown in Table 2 and plate materials of kinds shown in Table 2 were each used as an aluminum material. An aluminum piece having a size of 50 mm×50 mm×10 mm was cut out from each of the plate materials, and was subjected to mirror treatment by a mirror processing method shown in Table 2 (buff polishing) until a surface roughness Rt of less than 200 nm was achieved. The resultant aluminum piece after mirror treatment was subjected to pre-treatment for forming a pre-film under the treatment conditions shown in Table 2, and as well, subjected to anodic oxidation treatment at a target voltage under the treatment conditions shown in Table 2, followed by washing with water and drying. Thus, aluminum pieces (test pieces) after anodic oxidation treatment of Comparative Examples 1 to 10 were obtained.

[Evaluation of Crystal Grain Pattern Through Surface Observation]

The test pieces obtained in Comparative Examples 1 to 10 were each evaluated for its crystal grain pattern through surface observation in the same manner as in Examples described above. The results are shown in Table 2.

TABLE 1 Pre-treatment Aluminum material Kind of Al electrolytic purity Crystal solution and Electrical Example or grain treatment Voltage Time quantity No. kind (μm) Mirror treatment temperature (V) (min) (C/cm²)  1 99.99% 2,500 Buff polishing 15 wt % 5 1.4 0.1 Al Sulfuric acid (18° C.)  2 A1050 200 Buff polishing 15 wt % 5 1.3 0.1 Sulfuric acid (18° C.)  3 A5052 300 Buff polishing 15 wt % 5 1.3 0.1 Sulfuric acid (18° C.)  4 A7072 1,000 Buff polishing 15 wt % 5 1.2 0.1 Sulfuric acid (18° C.)  5 99.99% 2,500 Electropolishing 15 wt % 5 1.4 0.1 Al Sulfuric acid (18° C.)  6 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.)  7 99.99% 2,500 Chemical 15 wt % 5 1.4 0.1 Al polishing Sulfuric acid (18° C.)  8 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.)  9 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 10 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 11 99.99% 2,500 Cutting 15 wt % 5 0.7 0.05 Al processing Sulfuric acid (18° C.) 12 99.99% 2,500 Cutting 15 wt % 5 14 1 Al processing Sulfuric acid (18° C.) 13 99.99% 2,500 Cutting 2 wt % Oxalic acid 5 13 0.1 Al processing (10° C.) 14 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 15 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 16 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 17 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 18 99.99% 2,500 Cutting 15 wt % 0→5 2.5 0.1 Al processing Sulfuric acid (18° C.) 19 99.99% 2,500 Cutting 15 wt % 5→3 2.0 0.1 Al processing Sulfuric acid (18° C.) 20 99.99% 2,500 Buff polishing 15 wt % 3 2.0 0.1 Al Sulfuric acid (18° C.) 21 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 22 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) 23 99.99% 2,500 Cutting 15 wt % 5 1.4 0.1 Al processing Sulfuric acid (18° C.) Anodic oxidation treatment Kind of electrolytic Evaluation solution and Target Electrical through Example treatment voltage Time quantity surface No. temperature (V) (min) (C/cm²) Remark observation  1 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.)  2 15 wt % 15 33 20 *1 ◯ Sulfuric acid (18° C.)  3 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.)  4 15 wt % 15 30 20 *1 ◯ Sulfuric acid (18° C.)  5 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.)  6 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.)  7 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.)  8 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.)  9 15 wt % 15 10 5 *1 ◯ Sulfuric acid (18° C.) 10 15 wt % 15 70 40 *1 ◯ Sulfuric acid (18° C.) 11 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.) 12 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.) 13 2 wt % Oxalic acid 50 45 20 *1 ◯ (10° C.) 14 2 wt % Oxalic acid 50 45 20 *1 ◯ (10° C.) 15 2 wt % Oxalic acid 50 45 20 *2 ⊚ (10° C.) 16 2 wt % Oxalic acid 5→50 55 20 *2 ⊚ (10° C.) 0.5 wt % Oxalic 50→100 *3 acid (10° C.) 17 4 wt % 40 60 20 *1 ◯ Phosphoric acid (20° C.) 18 15 wt % 15 35 20 *4 ◯ Sulfuric acid (18° C.) 19 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.) 20 15 wt % 15 35 20 *1 ◯ Sulfuric acid (18° C.) 21 15 wt % 15 12 6 *1 ◯ Sulfuric acid (18° C.) 22 15 wt % 15 20 2 *1 ◯ Sulfuric acid (18° C.) 23 15 wt % 15 4 2 *5 ◯ Sulfuric acid (18° C.) (Note) *1: The target voltage is applied from the beginning. *2: The voltage is increased from 5 V to 50 V at a rate of 0.5 V/sec. *3: The voltage is increased from 50 V to 100 V at a rate of 0.5 V/sec. *4: The voltage is intermittently increased from 0 V to 15 V. *5: The pre-film partial dissolution treatment is performed through immersion in phosphoric acid between the pre-treatment and the anodic oxidation treatment.

TABLE 2 Pre-treatment Aluminum material Kind of Al electrolytic purity Crystal solution and Electrical Comparative or grain Mirror treatment Voltage Time quantity Example No. kind (μm) treatment temperature (V) (min) (C/cm²) 1 99.99% 2,500 Buff — — — — Al polishing 2 A1050 200 Buff — — — — polishing 3 A5052 300 Buff — — — — polishing 4 A7072 1,000 Buff — — — — polishing 5 99.99% 2,500 Buff 15 wt % 10 0.5 0.1  Al polishing Sulfuric acid (18° C.) 6 99.99% 2,500 Buff 15 wt %  5 0.3 0.02 Al polishing Sulfuric acid (18° C.) 7 99.99% 2,500 Buff — — — — Al polishing 8 99.99% 2,500 Buff — — — — Al polishing 9 99.99% 2,500 Buff 2 wt % Oxalic acid 0→5 2   0.02 Al polishing (10° C.) 10  99.99% 2,500 Buff 15 wt %   6.5 1   0.1  Al polishing Sulfuric acid (18° C.) Anodic oxidation treatment Kind of electrolytic Evaluation solution and Target Electrical through Comparative treatment voltage Time quantity surface Example No. temperature (V) (min) (C/cm²) Remark observation 1 15 wt % 15 35 20 *1 X Sulfuric acid (18° C.) 2 15 wt % 15 33 20 *1 X Sulfuric acid (18° C.) 3 15 wt % 15 35 20 *1 X Sulfuric acid (18° C.) 4 15 wt % 15 30 20 *1 X Sulfuric acid (18° C.) 5 15 wt % 15 35 20 *1 X Sulfuric acid (18° C.) 6 15 wt % 15 35 20 *1 X Sulfuric acid (18° C.) 7 2 wt % Oxalic acid 50 45 20 *1 X (10° C.) 8  4 wt % 40 60 20 *1 X Phosphoric acid (20° C.) 9 2 wt % Oxalic acid 50 45 20 *6 X (10° C.) 10  15 wt % 15 35 20 *1 X Sulfuric acid (18° C.) (Note) *1: The target voltage is applied from the beginning. *6 : The voltage is intermittently increased from 0 V to 50 V. 

1. An anodic oxidation treatment method for an aluminum material for subjecting an aluminum material comprising aluminum or an aluminum alloy to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a target voltage of 10 V or more, to form a porous anodic oxide film on a surface of the aluminum material, the method comprising, as pre-treatment of the anodic oxidation treatment, subjecting the aluminum material to anodic oxidation treatment in a treatment bath of a polybasic acid aqueous solution under a treatment condition of a voltage of 6 V or less until an electrical quantity reaches 0.05 C/cm² or more, to form a porous pre-film on the surface of the aluminum material.
 2. An anodic oxidation treatment method for an aluminum material according to claim 1, wherein a crystal grain in the aluminum material has a size of 100 μm or more.
 3. An anodic oxidation treatment method for an aluminum material according to claim 1 or 2, wherein the aluminum material has a surface subjected to mirror treatment.
 4. An anodic oxidation treatment method for an aluminum material according to claim 3, wherein the mirror treatment is performed by any mirror processing selected from buff polishing, electropolishing, cutting processing, and chemical polishing.
 5. An anodic oxidation treatment method for an aluminum material according to claim 1, wherein the treatment bath to be used in the pre-treatment and the treatment bath to be used in the anodic oxidation treatment comprise aqueous solutions of the same polybasic acid.
 6. An anodic oxidation treatment method for an aluminum material according to claim 1, wherein the treatment bath to be used in the pre-treatment and the treatment bath to be used in the anodic oxidation treatment comprise aqueous solutions of different polybasic acids.
 7. An anodic oxidation treatment method for an aluminum material according to claim 1, the method further comprising, when performing the anodic oxidation treatment of the aluminum material, performing pre-film removal treatment for removing the porous pre-film formed in the pre-treatment.
 8. An anodic oxidation treatment method for an aluminum material according to claim 7, wherein the pre-film removal treatment is performed during or after the anodic oxidation treatment.
 9. An anodic oxidation treatment method for an aluminum material according to claim 8, wherein the pre-film removal treatment during the anodic oxidation treatment comprises, when performing the anodic oxidation treatment of the aluminum material after the pre-treatment, performing the anodic oxidation treatment with an electrical quantity 50 times or more as large as the electrical quantity applied in the pre-treatment, to dissolve the porous pre-film formed in the pre-treatment in the treatment bath of the anodic oxidation treatment to thereby remove the porous pre-film.
 10. An anodic oxidation treatment method for an aluminum material according to claim 8, wherein the pre-film removal treatment after the anodic oxidation treatment comprises immersing the aluminum material after the anodic oxidation treatment in an acid or alkaline aqueous solution, to chemically dissolve the porous pre-film remaining on a surface after the anodic oxidation treatment to thereby remove the porous pre-film.
 11. An anodic oxidation treatment method for an aluminum material according to claim 1, the method further comprising, when performing the anodic oxidation treatment of the aluminum material after the pre-treatment, performing pre-film partial dissolution treatment for dissolving the porous pre-film formed on the aluminum material after the pre-treatment under treatment conditions in which 10% or more of a wall thickness of the porous pre-film remains.
 12. An anodic oxidation treatment method for an aluminum material according to claim 11, wherein the partial dissolution treatment of the porous pre-film comprises treating the aluminum material after the pre-treatment under treatment conditions preliminarily determined by preparing a test sample which is substantially the same as the aluminum material after the pre-treatment, and determining treatment conditions in which 10% or more of a wall thickness between pores of the pre-film remains through use of the test sample. 